Ian McEwan is a very successful novelist, but he hasn't let it go to his head. "Most of humanity gets by without reading novels or poetry," he says evenly, stretching out his long frame on a sofa in his London town house. "And no one would deny the richness of their thoughts." Most of humanity probably won't read his new novel, Saturday (Doubleday; 289 pages), which arrives in stores next week. But the sizable part that does will gain definite advantages in the richness of its thinking about brain surgery, the war in Iraq, the psychic burden of life after Sept. 11 and how it feels to be sucker-punched by an excitable creep.